This invention lies in the field of burners for for burning low BTU gas fuel. More particularly, it concerns a burner system that can accept either or both low BTU gas and high BTU gas, in any selected ratio, to burn effectively in a furnace.
As far as is known, prior art for the burning of low BTU (lean) gases as fuels for conservation of more standard fuels, has made use of unpreximed with air (raw gas) fuel burning principles. Where air can be premixed with fuel before burning the fuel burning is greatly accelerated and improved. The burning of low BTU gas fuel without air premixture leaves much to be desired in the burning process. Raw gas, or unpremixed fuel burning has been the sort of the prior art because it was felt that lean gas would be so diluted by premixture that it would not burn stably or would not burn at all.
We have made a study of a 90 BTU/cu. ft. lean gas which is 28% CO and 72% inert gases, and have invented a burner structure which premixes air with the 28% CO gases to the degree that close to theoretical air is pesent, as the premixed gas-air is discharged for fuel burning in the combustion zone, which is immediately downstream of the gas-air mixture discharge device. Results of the research have proved that air-gas premisture for lean gas combustion is not only feasible but is very advantageous. This special advantage results from stable burning, because of notably increased speed of burning and a more sharply defined combustion zone.
One result of the research is proof that, for a critical service, lean gas can be used as a premix fuel, whereas, when burned in an unpremixed-with-air burner, does not have suitable burnin characteristics for the required service. However, this alone is not enough for the solution of a combustion problem since, because of the low heating value of the lean gases, there may not be enough of them for supply of a required quantity of heat. Because of this, it must be possible to burn both the lean gases and a supplemental fuel supply based on a much richer fuel gas. The supplement gas can be methane (which has 910 BTU per cu. ft. LHV), or natural gas, or equivalent. Both lean and rich gases must be burned in the same burner structure in this case. Such a structure has been proven and is the basis of this invention.
An additional reason for dual fuel operation is that the lean gases are generally products of process operation. Prior to initiation of stable operation there are no lean gases available to burn. In order to establish stable operation from a cold start, a so-called "standard" fuel must be burned for heat production, to make the lean gases available for their fuel value. This requires the use of a common air aspirator and premixer for both fuels. Also, the burning apparatus must be suited to either or both gaseous fuels as required for adequate release of heat, and according to fuel availability.